This application claims priority to an application entitled xe2x80x9cChannel Communication Device and Method for Mobile Communication System Using Transmission Antenna Diversityxe2x80x9d filed in the Korean Industrial Property Office on Aug. 20, 1998 and assigned Serial No. 98-34187, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates generally to a mobile communication system, and in particular, to a channel communication device and method using transmission antenna diversity.
2. Description of the Related Art
CDMA (Code Division Multiple Access) mobile communication systems have developed a voice-based mobile communication standard called IMT-2000 which is capable of transmitting data at a high rate as well as voice. The IMT-2000 standard provides services such as high-quality voice, moving pictures and Internet search. In a CDMA mobile communication system, a communication path between a mobile station and a base station is classified into a forward link, where data is transmitted from the base station to the mobile station, and a reverse link, where data is transmitted from the mobile station to the base station.
The capacity of the forward link requires much higher gain as the traffic increases. If the mobile station moves slowly, transmission antenna diversity on the forward link has a gain of about 1-7 dB as compared with non-transmission antenna diversity. This means that the capacity of the system can be increased two or three times. The performance of the system is greatly improved when the receiver of the mobile station cannot obtain sufficient path diversity and when the moving velocity of the mobile station is low.
Transmission antenna diversity indicates that a path over which a signal transmitted from the base station reaches the mobile station is multiplexed to raise the reliability of a received signal with respect to the same transmission signal power of the base station. In this context, the path means not only a spatial path, but a physical path which can transmit a signal from a transmitting side to a receiving side. For instance, the path can be the direction of a transmission antenna, polarization of a transmission signal, position of a transmission antenna, different carrier wave frequencies on a frequency axis, and different transmission time points on a time axis. As an example of transmission diversity, there is TSTD (Time-Switched Transmission Diversity) in which the base station has a plurality of antennas and the signal to be output through a transmitter is selected by a switch, thereby multiplexing a path reaching the mobile station from the base station.
Referring to FIG. 1, which illustrates a conventional TSTD transmitter, a signal mapping circuit 111 receives a combined signal of encoded user data and long codes, and maps the level of the received signal. That is, the signal mapping circuit 111 converts an input signal xe2x80x9c0xe2x80x9d into xe2x80x9c+1xe2x80x9d and xe2x80x9c1xe2x80x9d into xe2x80x9cxe2x88x921xe2x80x9d. A serial-to-parallel converter 113 converts a serial signal output from the signal mapping circuit 111 into a parallel signal and separately outputs odd signals and even signals. Multipliers 115 and 117 respectively multiply the even signals and odd signals output from the serial-to-parallel converter 113 by an orthogonal code Wm. The multipliers 115 and 117 are to modulate (or spread) a desired user signal to the orthogonal code. A Walsh code may be used for the orthogonal code. A complex PN (Pseudo Noise) spreader 119 spreads the orthogonally modulated signals output from the multipliers 115 and 117 by using PN sequences PNI and PNQ. A controller 120 generates a switching control signal for distributing a transmission signal to a plurality of antennas by using TSTD.
A switch 121 switched by the switching control signal provided from the controller 120 has a common terminal connected to I-channel and Q-channel spread signal output terminals of the complex PN spreader 119, a first output terminal connected to LPFs (Low-Pass Filters) 123 and 125, and a second output terminal connected to LPFs 127 and 129. The switch 121 switches the spread signal output from the complex PN spreader 119 to either one of two pairs of the LPFs 123 and 125 and the LPFs 127 and 129 by the switching control signal.
The LPFs 123 and 125 low-pass filter the I-channel and Q-channel PN spread signals output through the switch 121. Multipliers 131 and 133 respectively multiply the outputs of the LPFs 123 and 125 by carrier wave frequency signals to output up-converted frequency signals. An adder 141 adds signals output from the multipliers 131 and 133 to each other. The added signal is transmitted via transmission antenna A.
The LPFs 127 and 129 low-pass filter the I-channel and Q-channel PN spread signals output through the switch 121. Multipliers 135 and 137 respectively multiply the outputs of the LPFs 127 and 129 by carrier wave frequency signals to output up-converted frequency signals. An adder 143 adds signals output from the multipliers 135 and 137 to each other. The added signal is transmitted via transmission antenna B.
In addition to TSTD, there is STD (Selective Transmission Diversity) in which the mobile station measures the power strength of pilots received from two transmission antennas, compares its relative rate with a threshold, and then periodically transmits an antenna selection signal to the base station. The base station then transmits data via the one antenna with better reception, as selected by the mobile station.
Referring to FIG. 2, which illustrates a conventional STD transmitter, a channel encoder 211 encodes data to be transmitted. A convolution encoder or a turbo encoder may be used for the channel encoder 211. An interleaver 212 interleaves symbols output from the channel encoder 211 to prevent a burst error. A multiplexer (MUX) 213 multiplexes a pilot symbol, a transmission power control bit (TPC), rate information (RI), and data interleaved by the interleaver 102. A serial-to-parallel converter 214 maps an input channel signal and converts the mapped signal into a parallel signal. For instance, the serial-to-parallel converter 214 converts an input signal xe2x80x9c0xe2x80x9d into xe2x80x9c+1xe2x80x9d and xe2x80x9c1xe2x80x9d into xe2x80x9cxe2x88x921xe2x80x9d and separately outputs in parallel odd signals and even signals. Multipliers 215, 225 and 235 respectively multiply the parallel channel signals output from the serial-to-parallel converter 214 by corresponding orthogonal codes. An adder 216 adds the orthogonally modulated signals output from the multipliers 215, 225 and 235 to each other. A complex spreader 217 spreads the output of the adder 216 by using PN sequences PNI and PNQ. A switch 218 switches the output of the complex spreader 217 by an antenna selection signal (AS) provided by a controller. The antenna selection signal is determined by an antenna switching command from the mobile station. Baseband filters 219 and 220 low-pass filter the I-channel and Q-channel PN spread signals switched by the switch 218.
FIG. 3 is a diagram illustrating the structure of signals exchanged between the base station and a mobile station in a conventional asynchronous mobile communication system that does not use transmission antenna diversity. Reference numeral 301 denotes a forward perch channel received by the mobile station (MS). The perch channel includes a search code for time synchronization. The search code is used to determine to which cell group a mobile station belongs in order to acquire slot sync and frame sync. That is, the mobile station acquires frame sync and slot sync using the search code of the perch channel, and obtains the dedicated physical channel information using a time offset provided from the base station. The mobile station measures the phase and power of the pilot symbols to estimate a channel condition. Reference numeral 302 indicates a forward dedicated physical channel (DPCH) received by the mobile station. The dedicated physical channel is a communication channel (or dedicated control channel) assigned exclusively to a subscriber. There is a time offset TFrame+TSlot between the perch channel and dedicated physical channel. Here, TFrame indicates an offset of a slot unit, and TSlot an offset of a symbol unit. Reference numeral 303 denotes a reverse dedicated physical channel transmitted by the mobile station. In the reverse dedicated physical channel, a pilot symbol and a TPC are time-multiplexed at each slot. There is an offset of 0.25 ms between the forward and reverse dedicated physical channels.
FIG. 4 is a diagram illustrating configurations of signals exchanged between a base station and a mobile station when the base station having multiple antennas transmits a dedicated physical channel using STD in a conventional asynchronous mobile communication system. Reference numeral 401 indicates a forward perch channel transmitted by antenna 1 and received by the mobile station, and reference numeral 411 a forward perch channel transmitted by antenna 2 and received by the mobile station. The perch channel includes a search code for time synchronization. The oblique lines represent pilot symbols. The mobile station measures the phase and power of the pilot symbols to determine which one of the two antennas has little channel distortion. The mobile station selects an antenna for transmitting the dedicated physical channel and informs the base station of the selected antenna. Reference numeral 402 indicates a forward dedicated physical channel transmitted via antenna 1 to the mobile station, and reference numeral 412 indicates a forward dedicated physical channel transmitted via antenna 2 to the mobile station. A time offset TFrame+TSlot exists between the perch channel and dedicated physical channel. Reference numeral 403 indicates a reverse dedicated physical channel transmitted by the mobile station. In the reverse dedicated physical channel, a pilot symbol and a TPC are time-multiplexed at each slot. Moreover, in the reverse dedicated physical channel, the TPC is punctured at the fourth slot and an antenna switching command (ASC) is inserted into the punctured position. The mobile station transmits the dedicated physical channel in which the ASCs are inserted to the base station. The base station then transmits the dedicated physical channel via a corresponding antenna selected by the ASC. In FIG. 4, the antenna is selected every 4 slots.
Consequently, in conventional STD, the perch channels in which the sync channels are time-multiplexed are consecutively transmitted by using different codes according to the antennas so that the mobile stations can distinguish between a plurality of antenna signals transmitted from the base station. Thus, since the perch channel is transmitted via two antennas, transmission power is increased. Therefore, the capacity of the base station is decreased, and the system is complicated since different pilots are used according to the antennas.
An object of the present invention is to provide a communication device and method for effectively transmitting a common channel (a perch channel and a common physical channel) used to select a transmission antenna by a base station, when the base station having a plurality of antennas transmits a dedicated channel by using STD in a CDMA communication system.
Another object of the present invention is to provide a communication device and method for controlling antenna switching in consideration of a time offset between channels in an asynchronous mobile communication system.
To achieve these and other objects of the present invention, there is provided a channel communication method for a mobile communication system using transmission antenna diversity, including the steps of: alternatively transmitting, at a base station, signals on a common channel via at least two antennas at first and second intervals; measuring, at a mobile station, the power of the received signals corresponding to the first and second intervals; selecting, at the mobile station, an antenna having stronger power by comparing the power of the signal corresponding to the first interval with the power of the signal corresponding to the second interval, and informing the base station of the selected antenna; and transmitting, at the base station, a dedicated channel via the selected antenna.